Vapor phase metal plating process

ABSTRACT

A process for the uniform evaporation of metals is accomplished by rotating a substantially cylindrical refractory element through a source of molten metal to coat the surface of said element with a thin film of molten metal, heating a portion of the molten metal coated surface of the refractory element to a temperature sufficient to vaporize at least a portion of the metal thereon. Condensing the metal vapors thus produced on a cooler substrate produces a solid metal film thereon. Passing the metal vapors into a cooler space containing an inert gas under low pressure or submicroscopic nucleic particles produces metal powders.

[ 5] Mar. 7, 1972 [54] VAPOR PHASE METAL PLATING PROCESS [72] lnventor: Robert 0. Lindblom, Walnut Creek, Calif.

[73] Assignee: The Dow Chemical Company, Midland,

Mich.

[22] Filed: July 18,1969

2|] Appl.No.: 848,136

[52] US. Cl. ..117/107.1, 75/.5 B, 75/.5 C, 117/106, 117/107, 118/49, 118/491 [51] Int. Cl. ..C23c 13/00, C230 13/02 [58] FieldoISearch ..1 17/106, 107, l07.l,93.3; 75/.5 B, .5 C

[56] References Cited UNlTED STATES PATENTS 3,046,936 7/1962 Simons, Jr ..1 17/933 X 8/1962 Allen et al ..75/.5 B 9/ 1964 Gatti Primary Examiner-Ralph S. Kendall Assistant Examiner-Kenneth P. Glynn Attorney-Griswold & Burdick and Raymond B. Ledlie [57] ABSTRACT A process for the uniform evaporation of metals is accomplished by rotating a substantially cylindrical refractory element through a source of molten metal to coat the surface of said element with a thin film of molten metal, heating a portion of the molten metal coated surface of the refractory element to a temperature sufficient to vaporize at least a portion of the metal thereon. Condensing the metal vapors thus produced on a cooler substrate produces a solid metal film thereon. Passing the metal vapors into a cooler space containing an inert gas under low pressure or submicroscopic nucleic particles produces metal powders.

1 Claim, 1 Drawing Figure PATENTEDHAR 7 I972 INVENTOR. Robe/"f0, L/ndb/am VAPOR PHASE METAL PLATING PROCESS BACKGROUND OF THE INVENTION The vapor phase coating of aluminum onto various substrates is a well known and widely practiced technique employed to make microminiature electronic components and optical equipment. It is likewise employed to apply thin aluminum-based coats on plastic film which is then slit into narrow strands to make metallic-appearing yarns. One of the major problems with presently available vapor phase aluminum plating processes, however, has been that the molten aluminum spatters during evaporation thereby burning holes in some materials. Another problem encountered in present evaporation methods is that the heating elements employed are either subject to localized corrosion due to solubility in the molten aluminum or such elements rapidly become embrittled or both.

SUMMARY OF THE INVENTION This invention relates to an improved process and apparatus for the evaporation of molten metals and more particularly relates to a process and apparatus for producing metal powders and for the evaporation and vapor phase plating of metals onto various substrates whereby contact of the substrate with molten metals is substantially eliminated.

It is an object of this invention to provide an improved method for thermally evaporating metals.

Another object of the invention is to provide an improved apparatus for the thermal evaporation of metals.

Still another object of the invention is to provide a method and apparatus to substantially eliminate the spattering of the molten metal during evaporation.

These and other objects and advantages of the present process will become apparent from a reading of the following detailed description.

It has now been discovered that metal may be evaporated in an evacuated chamber without spattering by rotating a substantially cylindrical refractory element through a bath or other source of molten metal to coat the cylindrical surface with a thin film of molten metal and heating at least a portion of such film by bombardment with sufficient high-energy electrons or by other means to increase the temperature of the metal film sufficiently to achieve a substantial evaporation rate. The metal vapors thus produced are then condensed in a space containing, for example, an inert gas under low pressure, to form a metal powder, or condensed to form a solid metal film on the surface of a cooler substrate present in the evaporation zone. A metal film produced in this manner is substantially uniform and free from the holes or irregular deposits which are produced when spattering occurs.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a diagrammatic side elevation of the apparatus of the invention showing one embodiment thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS In the FIGURE, an evacuated chamber 20 is provided. Within evacuated chamber 20, the surface of a refractory cylinder is rotated through molten aluminum 11 contained in vessel 12. In passing through the molten aluminum 11, the surface of the refractory cylinder 10 becomes coated with a thin layer of aluminum 17. The accelerated electron beam 13 from high-energy electron source 14 strikes that portion of the thin molten aluminum film 17 contained on the upper surface of the refractory cylinder 10 thereby heating and vaporizing the aluminum. Aluminum vapors 15 pass upward and continuously contact a moving film of substrate 16 to deposit a plate or coating of aluminum metal on such film as it passes from feed roll 19 to takeup roll 18.

Alternatively, fine aluminum powders are prepared by omitting rolls 18 and 19 and substrate 16 and passing aluminum vapor into a partially evacuated chamber 20 which is at a temperature below the melting point of the aluminum. The aluminum vapor passes into this chamber, condenses into an extremely fine powder and falls to the bottom of evacuated chamber 20 where it is collected and recovered by suitable means.

For condensation of the metal vapors to be achieved in the free space of evacuated chamber 20, it is necessary that sufficient nuclei be present in such chamber to provide a site or substrate on which the metal vapors can condense. Such nuclei may be, for example, fine metal particles which are .formed on cooling of metal vapors by contact 'with inert gas molecules or are introduced as such into chamber 20 by any suitable means. In the case of an inert gas such as argon, the presence of sufficient argon to exert a pressure of 10-200 microns is usually adequate to provide for the formation of the nuclei necessary to initiate condensation of the metal vapors.

The substantially cylindrical refractory element for use herein has at least an outer surface of a refractory material which is wetted by molten aluminum and which is not excessively corroded, melted or embrittled thereby. Suitable refractory materials include dense carbon, boron nitride, the metals molybdenum, titanium and tantalum together with their borides and nitrides and metallic tungsten if supported on another high temperature resistant material such as carbon. The outer surface of the cylinder of refractory material is preferably sufficiently smooth to provide a relatively uniform layer of molten metal on its surface after passing through the molten metal bath. For most applications dense carbon provides the preferred refractory material. This refractory element may be rotated at substantially any speed which will not throw molten metal from the surface of the element. The preferred speed of rotation depends on a number of factors including the rate of heat input to the molten metal surface to cause evaporation thereof. In general, however, a rotational speed of 2 to 4 revolutions per minute has been found suitable.

The use of such a rotating refractory element provides several advantages not heretofore available in processes for the vapor deposition of metals. For example, since the process involves no flow of molten metals across the surface of the evaporator element, the erosion problem usually caused thereby is eliminated. Likewise, the uniform contacting of the refractory evaporator element by molten metal and the uniform heating of such element causes any chemical attack by the molten metal to occur uniformly over the entire surface of such element and thereby substantially eliminates the mechanical stress and splitting of the element as is frequently encountered in unsymmetrical evaporators.

By employing the apparatus and process of this invention, mixtures or alloys of metals are effectively vapor deposited on substrates by heating the evaporation zone of the refractory element to dryness. In this manner, since little or no fractionation occurs, a substantially uniform metal composition is deposited on the substrate. Evaporation to dryness is easily accomplished in accordance with this invention by employing a relatively slow rate of rotation of the refractory element and employing a relatively high-intensity beam of electrons to vaporize the metal.

As to the metal to be evaporated, any metal may be employed which is volatile under high vacuum at a temperature below about 3,000" C. While aluminum is the preferred metal for use herein, metals such as nickel, copper, iron and titanium may also be employed. Likewise, mixtures and alloys of two or more metals may be employed. For example, it is usually desirable to add a few percent of titanium to a molten aluminum bath to inhibit aluminum carbide formation when a carbon cylinder is employed.

During the evaporation process, the molten metal bath is maintained at a temperature above its melting point but below the evaporation temperature of the metal employed. When aluminum is used as the metal, it is desirable to hold the molten aluminum bath at a temperature of between about the melting point and about l,800 C. with a temperature of between about 700 and l,l00 C. being usually preferred.

For the vaporization of most metals, it is necessary to employ a high vacuum. In general, a vacuum of at least about 10- Torr achieves good evaporation rates at practical temperatures.

Localized heating of the molten metal film may be achieved by any suitable means including high-energy electron sources capable of providing and focusing accelerated electrons, localized high-frequency currents generated inductively or capacitatively, focused high-intensity radiation generated by high-temperature filaments located internally or externally with respect to the rotating cylinder, and the like.

When a high-energy electron source is employed as the heating means, sufficient high-energy electrons are contacted with the molten aluminum film and the surface of the refractory cylinder to supply the heat necessary to evaporate the metal at the rate desired and to compensate for the radiation and conduction losses. For vaporization of aluminum, a temperature of from about l,l to about l,800 C. is usually employed with from about 1,20() to about l,600 C. being preferred under a vacuum of at least 10 Torr. The vaporization temperature of a given metal or alloy will vary, of course, depending on the composition of the metal or alloy and upon the pressure on the system. Sufiicient heating, however, vaporizes the aluminum or other metal and the vapors generally pass radially outward to contact the substrate to be plated.

Substrates which may be plated with a metal by the process of this invention include sheets, strips and foils of metals, glass, ceramics, papers, textiles and organic polymers. When contacted with the metal vapor, the surface temperature of the substrate must be sufficiently low to permit the condensation of the metal vapor thereon to produce a solid metal film. Substrates to be plated are therefore usually not heated much beyond the ambient temperature of the atmosphere in theevacuated vapor phase plating apparatus.

If the molten metal source is grounded, the source of makeup metal need not be electrically insulated in order to permit the adding of additional metal during operation of the high-voltage electron source. It is sometimes preferred, however, to ground the negative side of the power supply to the high-energy electron source. The molten metal is then positive with respect to both the electron source and to ground and the electrons therefore flow only to the molten metal film. When this arrangement is employed, however, additional metal added to the molten metal bath must be electrically insulated from ground or the electron source must be shut down during the addition of metal to the molten metal bath.

The following examples are for the purpose of further illustrating the invention but are not to be construed as limiting to the scope thereof.

EXAMPLE 1 In a vacuum chamber, a heated carbon trough was filled with a molten aluminum alloy containing 5 percent by weight impingement of about 1,300 C. causing vaporization of the aluminum alloy at a rate sufficient to deposit 300 A/minute of such alloy on the polypropylene sheet. About 10 to 15 weight percent of the molten aluminum presented to the electron beam was evaporated. No spattering occurred and a smooth adherent coating of aluminum was produced on the substrate.

In a control run, an electron beam of the same magnitude was applied to the molten aluminum alloy bath without the rotatin carbon cylinder. Substantially the same evaporation rate 0 aluminum alloy was achieved ut very severe spattering was observed on the metallized polypropylene film.

EXAMPLE 2 In a manner of Example 1, a film of copper was deposited on polyethylene sheeting at a rate of about 400 A/minute by employing a bath of molten copper at a temperature of about 1,200 C. and a molybdenum cylinder rotating at a speed of about 3 r.p.m.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that 1 limit myself only as defined in the appended claims.

I claim:

1. The process for providing a metal coating on a substrate which comprises: heating a metal above its melting point and below its vaporization point to provide a liquid metal and maintaining it in the liquid state in a vacuum of at least about 10' Torr; rotating a generally cylindrical refractory element so that during a part of the rotation period a first portion of the periphery thereof is submerged below the surface of the liquid metal, thereby to provide an adhering film of metal onto said portion, and during a subsequent part of the rotation period said first portion of the periphery is exposed above said surface and a succeeding portion of the periphery is submerged in the liquid metal;

impinging a stream of electrons from a high-energy electron source onto a selected site of the film adhering to the exposed portion of the periphery whereby at least a portion of the metal film is vaporized;

moving the substrate, at a temperature below the melting point of said metal and at said low positive pressure, across the path of the vaporized metal whereby the metal contacts and adheres to the substrate to provide a substantially continuous coating of controlled thickness thereon. 

